1. Field
This invention relates to magnetoresistive heads using an magnetoresistive effect by which information is reproduced from a magnetic recording medium, and magnetic heads by which recording and reproduction are carried out.
2. Background
Usually, a magnetic head provided in a hard disk drive (HDD) includes: a writing head for writing information as a magnetization signal onto a recording medium (hard disk); and a reproducing head for reading the signal recorded as a magnetization signal on the recording medium. Since the reproducing head is constructed by a stack including a plurality of magnetic thin films and non-magnetic thin films and reads the signal by using the magneto-resistive effect, it is called a magneto-resistive effect head. There are a few kinds of stacking structures of the magneto-resistive effect head and they are classified into an AMR head, a GMR head, a TMR head, and the like in accordance with a principle of magneto-resistance which is used. More particularly, an input magnetized field entered from the recording medium into the reproducing head is extracted as a voltage change by using an AMR (Anisotropy Magneto-Resistive effect), a GMR (Giant Magneto-Resistive effect), CPP-GMR (Current Perpendicular Plane-GMR effect) or a TMR (Tunneling Magneto-Resistive effect).
In the stack layer of the reproducing head, a magnetic layer whose magnetization is rotated by receiving the input field from the recording medium is called a free layer. To suppress various noises such as a Barkhausen noise and the like or to control an asymmetry output, it is important to convert the free layer into a single domain in a track width direction. If the free layer has a magnetic domain without being converted into a single domain, the free layer receives the input magnetized field from the recording medium, so that a domain wall movement occurs and becomes a cause of noise.
As a method of the magnetic domain control for converting the free layer into the single domain, for example, as disclosed in JP-A-3-125311, there is a method whereby magnetic domain control layers including magnet layers are arranged at both ends of the free layer, and a magnetic field which is caused in the track width direction from the magnet layers is used. FIG. 8 shows a schematic diagram which is obtained when a magneto-resistive head subjected to the magnetic domain control by such a method is seen from an air bearing surface. A free layer 2 is formed via a spacer 3 over a soft magnetic layer (called a pinned layer) 4, and magnetization of the soft magnetic layer 4 has been fixed by an antiferromagnetic layer 5. A cap layer 1 is formed on the free layer 2. A width of free layer 2 is called a track width Twr. Both ends of the stack of the antiferromagnetic layer 5 are shaved from the cap layer 1 by ion milling or the like, so that a device has a trapezoidal shape when it is seen from the air bearing surface as shown in FIG. 8. A structure of the head of FIG. 8 is characterized in that magnetic domain control layers 7 including magnet layers are arranged to both ends of the device via seed layers 6. According to such a structure, a magnetization distribution of the free layer 2 is controlled by using a magnetic field which is developed from the magnetic domain control layers 7, and the free layer is converted into the single domain.
As another method of the magnetic domain control, for example, as shown in U.S. Pat. No. 4,663,685, there is a method whereby antiferromagnetic films are stacked on both ends of a free layer and an exchange coupling between the antiferromagnetic film and the free layer is used. FIG. 9 shows a schematic diagram which is obtained when a magneto-resistive effect head subjected to the magnetic domain control by such a method is seen from an air bearing surface. A structure of the head of FIG. 9 is characterized in that the free layer 2 is formed via the spacer 3 over the soft magnetic layer (called a pinned layer) 4 whose magnetization has been fixed by the antiferromagnetic layer 5, and antiferromagnetic films 12 are stacked at both ends of an upper portion of the free layer 2.
A magnetic domain control is performed by an exchange interaction which acts between the antiferromagnetic film 12 and free layer 2. The free layer 2 is formed so as to be wider than the width of track written on the recording medium and has a shape such that end regions are fixed. According to such a structure, therefore, a record is read by a portion Tw (of the free layer 2) between the antiferromagnetic films 12 (such a portion is called a sensing region). A lead layer 10 is stacked over the upper surface of the antiferromagnetic film 12 via a seed layer 11. It is not always necessary to form the seed layer 11.
As shown in JP-A-11-203634, there is also a method of stacking an antiferromagnetic layer having a uniform thickness onto the whole surface of a free layer, or the like. However, since a track width of the present head is very narrow, there is a fear that, if the whole surface of the free layer is fixed by a uniform magnetic field, sensitivity deteriorates and a magnetic domain control field of a track end portion where the magnetic domain control is particularly necessary is contrarily insufficient. As shown in JP-A-2001-84527, a method whereby a magnetic domain control layer is constructed by a stack layer of a layer of high coercivity and at least one of a ferromagnetic layer and an antiferromagnetic film has also been proposed.
Further, as proposed, for example, by Japanese Patent Laid-Open No. 282618/1997, Japanese Patent Laid-Open No. 53716/1999 or U.S. Pat. No. 5,739,990 is a system in which, to realize high reproducing output, a dead zone area, not to be used for reading, is created at the end portions of the free layer in the track in a widthwise direction by making the distance between the leads smaller than the track width of the free layer, since the magnetic orientation of the free layer is made difficult to rotate by the intensity of the magnetic fields generated by the lamination.
Each of the above magnetic domain control structures has the following problems. According to the magnetic domain control system as shown in FIG. 8 such that the magnetic domain control layers comprising the magnet layers are arranged on both sides of the free layer, since a magnetic field which is developed at an interface where the magnetic domain control layer and the free layer are come into contact with each other is too strong, a region (dead region) where the magnetization of the free layer is hard to be rotated with respect to the medium field is caused. To reduce the dead region, it is sufficient to weaken a magnetic domain control force by simply thinning the thickness of magnet layer or by another method. In case of using such a method, however, since an inconvenience such that the Barkhausen noise or an output signal instability is contrarily caused, asymmetry of an output increases, or the like occurs, the magnetic domain control force of a certain extent is necessary.
When a recording density of the recording medium is large and the track width Twr which is defined by the width of free layer is wide, since a ratio of the dead region which occupies the track width Twr is small, such an influence does not cause a large problem. However, the track width Twr is decreasing more and more in accordance with a recent extreme increase in recording density. Therefore, the ratio of the dead region which occupies the track width Twr is increasing. Unless some countermeasures are taken, it is very difficult to assure enough sensitivity of the head without deteriorating characteristics of noises or the like.
As one of the countermeasures, a method whereby an interval between leads is set to be smaller than the interval of the track width Twr and the portion of the dead region of low sensitivity is not used for reading, thereby enabling a high reproduction output to be obtained has been proposed in, for example, JP-A-9-282618 or U.S. Pat. No. 5,739,990. However, according to such a method, there is a problem such that since a magnetic domain control force which is applied to the end portion of the free layer locating under the lead is weak, a side reading increases.
According to the magnetic domain control method as shown in FIG. 9 whereby the antiferromagnetic film is arranged, since the coupling field acts only on the portion where the antiferromagnetic film and the free layer are in contact with each other, the problem of the dead region as mentioned above does not occur. It is advantageous for realization of a narrow track. However, the exchange coupling field between the antiferromagnetic film and the free layer is weaker than that in case of using the magnet layers and it is insufficient as a magnetic domain control force.
When the apparatus such as an HDD or the like operates, although the magneto-resistive effect device generates heat, the exchange field of the antiferromagnetic film is deteriorated by the heat. Therefore, a problem such that the edge region of the free layer which ought to have been fixed also has sensitivity, the side reading occurs, the record on the adjacent track is read, and an error rate deteriorates occurs.
In addition to the problems relating to the dead zone area, it becomes a significant problem that the ferromagnetic film is overlaid on an upper surface of the element. The overlaid portion of the magnetic film generates a magnetic field in the opposite direction to the magnetic domain control magnetic field, and in the magnetic domain controlling magnetic field distribution across the track width (Twr) direction of free layer 2 shown in FIG. 19, rippling occurs in the vicinity of a portion in which magnetic film 7 is disposed, at end portions of free layer 2 in the track width (Twr) direction. Together with this rippling, the magnetic field which is applied to the end portion areas of free layer 2 in the track width direction is decreased. As a result, instability of the element is increased. In addition, to suppress the instability, magnetic film 7 must be made thicker than necessary, and, as a result, a magnetic field stronger than necessary is applied over the entire free layer 2 so that sensitivity is damaged. It is difficult to eliminate the overlay of magnetic film 7 completely due to the process employed. Thus it is necessary to use ingenuity with regard to the magnetic domain control structure.
In contrast, in a magnetic domain control method in which the antiferromagnetic film is disposed as shown in FIG. 9, coupling a magnetic field affects only the portion at which the antiferromagnetic film 12 is in contact with the free layer 2; therefore, the dead zone problem does not occur, and this method is also advantageous in making narrower tracks. Further, there is no effect of the overlay of the magnetic film on the element. However, the exchange coupling magnetic field of antiferromagnetic film 12 and free layer 2 is smaller as compared with a case in which magnetic film 7 is used.
As shown in FIG. 19, it is preferred that a magnetic field does not exist in a magnetic sensing region, but the magnetic field intensity is extremely small compared with the magnetic field of the end portions in the track width (Twr) direction of free layer 2 in a case where magnetic film 7 is used. Further, when an apparatus such as HDD (hard disk drive) is in use, the exchange magnetic field of antiferromagnetic film 12 and the end portion of free layer 2 deteriorates due to heat generated by the magnetoresistive element, and the magnetic domain controlling magnetic field is further decreased. Therefore, the problem arises that the end portion area of the free layer 2, which should be fixed, has sensitivity, and read drift occurs so that the record data of an adjacent track is read and the error rate is increased. Further, there is a process problem that, because the width Tw of the magnetic sensing region increases as the effective track width is broadened, the geometric track width Twr must be made smaller.